linux-kernel - [PATCH v2] selftests/bpf: Make x86 preempt

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Message-ID: <20260130021843.154885-1-changwoo@igalia.com>
Date: Fri, 30 Jan 2026 11:18:43 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	kernel-dev@...lia.com,
	bpf@...r.kernel.org,
	sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Changwoo Min <changwoo@...lia.com>
Subject: [PATCH v2] selftests/bpf: Make x86 preempt_count access compatible across v6.14+

Recent x86 kernels export __preempt_count as a ksym, while some old kernels
between v6.1 and v6.14 expose the preemption counter via
pcpu_hot.preempt_count. The existing selftest helper unconditionally
dereferenced __preempt_count, which breaks BPF program loading on such old
kernels.

Make the x86 preemption count lookup version-agnostic by:
- Marking __preempt_count and pcpu_hot as weak ksyms.
- Introducing a BTF-described pcpu_hot___local layout with
  preserve_access_index.
- Selecting the appropriate access path at runtime using ksym availability
  and bpf_ksym_exists() and bpf_core_field_exists().

This allows a single BPF binary to run correctly across kernel versions
(e.g., v6.18 vs. v6.13) without relying on compile-time version checks.

Signed-off-by: Changwoo Min <changwoo@...lia.com>
---
 .../testing/selftests/bpf/bpf_experimental.h  | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index a39576c8ba04..acabf869d2c4 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -614,7 +614,13 @@ extern int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,
 
 extern bool CONFIG_PREEMPT_RT __kconfig __weak;
 #ifdef bpf_target_x86
-extern const int __preempt_count __ksym;
+extern const int __preempt_count __ksym __weak;
+
+struct pcpu_hot___local {
+	int preempt_count;
+} __attribute__((preserve_access_index));
+
+extern struct pcpu_hot___local pcpu_hot __ksym __weak;
 #endif
 
 struct task_struct___preempt_rt {
@@ -624,7 +630,18 @@ struct task_struct___preempt_rt {
 static inline int get_preempt_count(void)
 {
 #if defined(bpf_target_x86)
-	return *(int *) bpf_this_cpu_ptr(&__preempt_count);
+	/* By default, read the per-CPU __preempt_count. */
+	if (bpf_ksym_exists(&__preempt_count))
+		return *(int *) bpf_this_cpu_ptr(&__preempt_count);
+
+	/* If __preempt_count does not exist, try to read preempt_count under
+	 * struct pcpu_hot. Between v6.1 and v6.14 -- more specifically,
+	 * [64701838bf057, 46e8fff6d45fe), preempt_count had been managed
+	 * under struct pcpu_hot.
+	 */
+	if (bpf_core_field_exists(pcpu_hot.preempt_count))
+		return ((struct pcpu_hot___local *)
+			bpf_this_cpu_ptr(&pcpu_hot))->preempt_count;
 #elif defined(bpf_target_arm64)
 	return bpf_get_current_task_btf()->thread_info.preempt.count;
 #endif
-- 
2.52.0